An apparatus that measures the electrophoretic mobility and the ζ (zeta) potential of particles that are contained inside a sample cell container and move under the influence of an electric field is called an electrophoretic mobility measurement apparatus.
With this measurement apparatus, a liquid (sample solution), in which a dispersion of particles is suspended, is contained in a cell-type test container having transparent walls (hereinafter referred to simply as “sample cell container”), light is irradiated on the sample solution, scattered light emitted from a certain region of the sample cell container is detected by a photodetector, the velocity of the particles is calculated by analyzing the frequency components of the scattered light, and a particle velocity distribution or an electrophoretic mobility distribution of the particles is calculated.
FIG. 10 is a diagram illustrating an electroosmosis phenomenon inside a conventional sample cell container. An electroosmotic flow is a movement, due to the presence of ions, of the liquid that supports the particle dispersion. The ions are transported by the electric field. The distribution of the ions is influenced by charges present on the walls of the sample cell container.
The liquid flows in one direction at locations near the inner peripheral walls of the sample cell container and flows in the opposite returning direction at a central region of the sample cell container. This is called the electroosmotic flow Uosm.
“Up” in FIG. 10 represents the net flow of the particles dispersed and suspended in the liquid. There exist planes at each of which a peripheral flow (a flow near a cell wall that flows in one direction) of the electroosmotic flow Uosm and a central counterflow (a flow flowing in the opposite direction at a central portion side when viewed from a cross section of the cell) contact and mingle so that the velocity of the liquid becomes zero. These planes are called “stationary planes.” In an electrophoretic mobility measurement method, it is considered preferable to attempt to perform a particle velocity distribution at the position of a stationary plane (see Patent Document 1).
In injecting a sample solution into a conventional electrophoretic mobility measurement cell, an electrode is set in an opening at one side of the cell and the opening is capped. The cell is then inverted and an appropriate amount of the sample solution is injected with a pipette, etc., from an opening at the other side while inclining the cell to avoid entry of bubbles. An electrode is then inserted in the opening at the other side and the opening is capped.
On the other hand, for samples mainly in bio-related fields and pharmaceutical-related fields, disposable cells, which are discarded after measurement because of adsorption of sample and attachment of contaminants on the glass cell, are adopted.